1. Field of the Invention
The present invention relates to an automatic assembling system of a galvanized steel sheet by spot welding where, in the field of automobiles and household electric appliances, spot welding is performed between the bonding or faying surfaces of members comprising at least two molded steel sheets through a galvanized layer to assemble these members as a structure.
2. Background Information
Galvanized steel sheets are being increasingly used in the field of automobiles and household electric appliances. However, problems of deterioration of the welding electrode of a welding machine have been occurring. Spot welding of a galvanized steel sheet requires a high weld current value and a long welding time (current conducting time), as compared to spot welding of a bare (non-coated) steel sheet, and consequently, deterioration of the welding electrode, that is, deformation of the welding electrode or formation of an alloy with zinc is promoted. If the welding electrode is deteriorated, the resulting weld will be unstable and, finally, a nugget will not be obtained. Accordingly, the welding electrode must be frequently replaced and the operating efficiency of the production line is reduced.
A small electric resistance value between bonding surfaces is considered to be a main cause for an early deterioration of the welding electrode. Noting this point, the inventors have recently proposed to interpose a resistance increasing material between the bonding surfaces of galvanized steel sheets which are to be jointed. Thus, the electric resistance between the bonding surfaces is increased to effect spot welding (See Japanese Unexamined Patent Publication Nos. 64-62284 and 64-62286. Japanese Patent Publication No. 5-85269 and U.S. Pat. Nos. 4,922,075 and 5,075,531). The result obtained by this attempt was an improved weld performance. That is, this welding method has the advantages that (1) the cost of electric power, (2) the chance of explosion, and (3) the welding stain are reduced, (4) marking is small, and (5) no burr occurs due to less consumption of energy in bonding. Also, with this method, melting of each welding electrode is significantly reduced, and thus smaller-sized spot welding machines can be used.
On the one hand, one problem with the spot welding of a galvanized steel sheet is that quality control is difficult. At present, an apparatus for monitoring a weld current and a voltage or resistance between electrodes is employed in quality control. Also, some nondestructive test methods have been proposed for welded portions. Furthermore, a peel test using a chisel has been widely carried out.
On the other hand, weld-bonding using bonding and welding together has recently attracted attention for the assembling of automobiles, and the range of its application is expanding gradually. While conventional spot welding results in point bonding, weld-bonding results in surface bonding. For this reason, weld-bonding can enhance bonding strength and rigidity and is effective in the weight reduction of the body of an automobile. Furthermore, the weld bonding method has advantages in that it results in an excellent vibration-impact characteristic, noise is reduced, and sealing performance is assured.
However, in the spot welding method where a resistance increasing material is interposed between bonding surfaces, apart from an experimental implementation or a case where the number of strike points or spots of welding is relatively small, when various kinds of galvanized steel sheets are used and parts with a complicated shape are welded many times for a short period of time, as in the case of automobiles, the method may be inefficient and unproductive.
More particularly, it is necessary in the aforementioned welding method that a resistance increasing material be accurately arranged at a fixed position on each bonding surface and also, just adjacent to the resistance increasing material, steel sheets electrically conduct current across the electrodes. In this case, however, the placement of the resistance increasing material cannot be confirmed from the outside. Furthermore, it is difficult in this welding method to maintain a good contact state between bonding surfaces because of the existence of the resistance increasing material. Particularly when the welding electrode is deteriorated, the current of the electrode tends to be unstable. There is the possibility that any of these disadvantages will lead to a reduction in productivity, and in spite of the remarkable improvement in welding performance, it was difficult to put this welding method to practical use in a mass production system.
Incidentally, with respect to quality control, spot welding is widely used in the assembly of automobile bodies, and it may be said that the quality of the automobile body is determined by whether the spot welding is good or bad. For example, the automobile body is constituted by 600 to 800 parts and most of them are assembled by spot welding. The number of welding strike points or spots reaches 3000 to 5000 per automobile. And, for various reasons, it is difficult to avoid occurrence of a welding defect in the manufacturing process. While the shape, marking, spatter, cracks, pit, and the like of a nugget are prescribed in judging the quality of the welded section of a galvanized steel sheet, in practice, it is considered most important to assure a proper nugget diameter. If the nugget diameter is insufficient, it will cause deterioration of an electrode or cable, reduction in the electric current value due to a fluctuation of the welding power supply voltage, and a misalignment between bonding surfaces. The fluctuation of the power supply voltage results from the simultaneous use of a plurality of spot welding machines, power use of other factories, and a difference between available day time and night time power supplies. And, in a galvanized steel sheet, the range of suitable electric current for spot welding is narrow, and a nugget may not be properly formed depending upon whether there is a variance in the electric current value. Therefore, there are good reasons why quality control is considered particularly important in the spot welding of galvanized steel sheets.
In the conventional monitoring apparatus described above, reliability is poor with respect to galvanized steel sheets, unlike the case of bare steel sheets, and consequently, there are many cases where welding lines are stopped due to problems. For this reason, the conventional monitoring apparatus is insufficient as far as improving the length of time a production line be continuously operated unmanned. In addition, in the aforementioned peel test method using a chisel, a sampling test is conducted, and if a defect is found, measures will be taken to check all previous products and carry out the spot welding again. Consequently, the labor costs of the test and the costs of abandoning the defective products have been excessive.
Under such circumstances, it is desirable that quality be guaranteed within a process, and the development of a monitor which checks all welded sections while they are welded is in demand. Furthermore, even in the case of the conventional weld bonding method, the aforementioned troubles resulting from deterioration of an electrode remain as they are.
Accordingly, it is a first objective of the present invention to improve the spot welding of a galvanized steel sheet using a resistance increasing material and to maintain excellent welding performance and high productivity under a mass production system.
It is a second objective of the present invention to solve problems in the quality control of welded sections and to overcome troubles associated with the quality of welding in advance under a mass production system. An in-process quality test is performed by checking all welded sections at the same time they are welded and also the troubles associated with the quality of welding are monitored in advance.
It is a third objective of the present invention, under a mass production system to apply a sealing function or an adhesive function to a welded section and to form the welded section such that assurance of sealing performance and enhancement in rigidity are high without increasing costs while achieving the first and second objectives.
In this research, the development of a resistance increasing material suitable for the present system was first attempted. Spot welding where the resistance increasing material is interposed between bonding surfaces has excellent welding performance, but has not yet been put to practical use, particularly under a mass production system. The main reason is that the resistance increasing material has been considered difficult to efficiently interpose between bonding surfaces. Therefore, in the present research, the development of a resistance increasing material which is easy to be interposed between bonding surfaces was considered. Particularly, a spacer, for example, alumina powder is incorporated into an adhesive material, and a necessary amount of the mixture is properly fed and arranged on a fixed layer on the bonding surface by means of an automatic coating machine. Also, a perforated tape, coated on both sides with an adhesive, can be used.
It should be noted that the resistance increasing material used in the present system should have excellent welding performance, a stable strike point over a long period of time, and a large resistance increasing effect in order to achieve the second objective of the invention, i.e., an in-process quality guarantee and adaptive successive automatic operation. A large resistance increasing effect causes an amount of reduction in an inter-electrode resistance value resulting from formation of a nugget to be increased, and consequently, it is conceivable that whether a nugget is a success or a failure can be accurately determined.
Furthermore, the resistance increasing material used in the present system must be one where a reduction in an adhesive force and in a sealing function does not occur by incorporation of the resistance increasing material in order to induce an adhesive effect to a welded section, which is the aforementioned third object. A suitable adhesive which achieves the aforementioned first and second objectives must be selected.
Taking these various points into consideration, the present inventors have conducted research and experiments seeking a resistance increasing material suitable for the present system.
An adaptive control system should be additionally discussed. The adaptive control system comprises a detection step, a calculation step, a step for judging whether a nugget is a success or a failure, a second recording step, an estimation step, and a control step, which are incorporated in the system of the present invention. This adaptive control system is aimed at the variation in the electrical characteristic between weld electrodes which occurs during successive stride points under a mass production system. The variation in the electrical characteristic includes, for example, an electric resistance value, i.e., a variation in an inter-electrode resistance value.
More specifically, the electrical characteristic between weld electrodes in the current conducting time of a weld current is detected in the detection step. Then, in the calculation step, the inter-electrode resistance value is calculated from the detected electrical characteristic, also a resistance value variation characteristic is calculated from the inter-electrode resistance value, and from this calculation result, an in-process quality guarantee is assured in the first and second judgment steps. Furthermore, in the second recording step, at least one kind of variation of the electrical characteristic, the inter-electrode resistance value, and the resistance value variation is recorded in detail during successive strike points. In the estimation step, the recorded data is analyzed and it is predicted from this result that a nugget is not formed as the welding electrode is deteriorated. Then, the weld conditions are altered in the control step. For the alteration of the weld conditions in the adaptive control step, there are, for example, several possibilities including grinding of the weld electrode, increasing welding pressure, extending current conducting time, and/or increasing the set electric current value. By automatically performing these controls, high productivity can be maintained and sound nuggets can be assured.
The aforementioned inter-electrode resistance value consists of a contact resistance between a welding electrode and a base member to be bonded, an inter-sheet resistance between the bonding surfaces of the base members, and an intrinsic resistance of the base members. The inter-electrode resistance value can be measured at a production line during welding. In the case of galvanized steel sheets, however, there is an established theory that the values give no information as to formation of nuggets. That is, the inter-sheet resistance disappears as a nugget is formed, but the current conducting time is long in the case of a normal welding method. For example, in a case where two galvanized steel sheets having a thickness of 0.8 mm are bonded together, a current conducting time of about 10 cycles is required. For this reason, the temperature of the base member rises during this welding, and consequently, the intrinsic resistance of the base member increases. The change in the inter-electrode resistance value where the inter-sheet resistance and the intrinsic resistance are summed does not always indicate the success or the failure of a nugget.
On the other hand, in the system of the present invention where a resistance increasing material is applied on each bonding surface the inter-sheet resistance value itself is high and also the current conducting time is short (about 3 cycles). Therefore, in the system of the present invention, there is the possibility that the disappearance of the inter-sheet resistance value resulting from formation of a nugget can be effectively detected. If such detection can be realized, the change in the inter-sheet resistance value can be examined in detail. Accordingly, not only the success or the failure of a nugget but also misformation of a nugget resulting from the deterioration of the welding electrode during successive strike points can be predicted, and the development of an adaptive control meeting the demand becomes possible.
Incidentally, there are a large number of influencing factors for the change in the inter-electrode resistance value of a galvanized steel sheet. In the galvanized steel sheet, if a weld current is conducted, zinc between the welding electrode and the galvanized steel sheet or between the bonding surfaces of the galvanized steel sheets will be first melted because its melting point is low. For the bonding surfaces, the melted zinc is evaporated and expanded and is discharged outside of an area where a nugget is formed. Then, the temperature of the bonding surface becomes higher than those of other sections, part of the steel sheet is melted and mixed, and a nugget is formed. When the nugget is formed, the inter-sheet resistance value will disappear.
Between the welding electrode and the galvanized steel sheet, a portion of zinc is melted and alloyed with the material of the welding electrode comprising copper or a copper alloy, and consequently, the electrode is gradually deteriorated. On the one hand, the temperature of the galvanized steel sheet continues to increase during the current conducting time due to its intrinsic resistance.
These phenomena are different in rate of progression depending upon the weld conditions, and consequently, the inter-electrode resistance value also varies in a complicated manner. The following are specific main factors which are considered to be related to the variation characteristic of the inter-electrode resistance value while the weld current is flowing.
1. Deterioration of Welding Electrode
If the welding electrode is deteriorated, problems will occur in the contact between the welding electrode and the galvanized steel sheet and the resistance value between the welding electrode and the base member will vary. As a result, the state of generation of heat will vary and therefore the melting and evaporation state of zinc will also vary. If zinc is melted, the resistance value will be greatly reduced. These phenomena influence each other and the inter-electrode resistance value varies in a complicated manner. On the other hand, between the bonding surfaces, the current density of the weld current is reduced due to the deterioration of the welding electrode, and the temperature rise in the bonding surfaces is delayed.
2. State of Galvanized Steel Sheets Which are Going to be Bonded Together
For workpieces of pressed members (members being fed on a production line), a problem of fit up occurs between the bonding surfaces. If fit up is insufficient, the contact area will become smaller. In this situation, the inter-sheet resistance value is high and insufficient bonding causes misconducting and irregular conducting of the weld current to occur. Also, since the welding current flows locally, the diameter of the nugget is insufficient.
3. Material and thickness of a Base Member or a Coated Layer
If a base member or a coated layer is thick, the temperature rise will be delayed and the inter-electrode resistance value will vary depending on the thickness of the base member or the coated layer.
4. The Number of Overlapped Steel Sheets
In a case where three or more galvanized steel sheets are overlapped and welded, the times when nuggets are formed are different at two or more bonding surfaces and the inter-electrode resistance value also changes.
5. Weld Current
The inter-electrode resistance values during current conducting are different between a case where the set value of a weld current is low and a case where the set value is high.
As described above, there are a large number of factors for affecting the variation in the inter-electrode resistance value of the galvanized steel sheet. Therefore, when manufacturing industrial products comprising a wide variety of members, particularly when manufacturing products by means of a mass production system, the nugget diameter and the variation in the inter-electrode resistance value need to be more accurately correlated for respective cases.
According to a first aspect of the invention, there is provided a spot welding system for assembling at least two initial members formed from a galvanized steel sheet into a structural member by spot welding bonding surfaces of the initial members through galvanized layers on the bonding surfaces by way of a spot welding machine having a pair of weld electrodes.
The system comprises the steps of:
placing a resistance increasing material at a predetermined position on the bonding surface of one of the initial members;
overlapping the other of the initial members on the one of the initial members while clamping the resistance increasing material between the initial members;
positioning a center axis passing through the pair of weld electrodes over substantially the center of the resistance increasing material clamped between the bonding surfaces to apply a predetermined pressure by the weld electrodes to the resistance increasing material and the initial members;
flowing a weld current having a predetermined value between the weld electrodes in a predetermined time;
detecting electric characteristics with respect to the weld electrodes in the predetermined time;
calculating an inter-electrode resistance based on the detected electric characteristics and calculating characteristics of resistance change based on the inter-electrode resistance;
determining success or failure in forming a nugget between the bonding surfaces by comparing the characteristics of resistance change with a predetermined standard;
automatically changing weld conditions upon the determination of failure in the determining step and primarily compensating the forming of the nugget;
comparing the predetermined standard with characteristics of resistance change additionally calculated after the compensating step and secondarily determining success or failure in forming the nugget;
recording the determination of failure in forming the nugget in the secondarily determining step;
continuously recording at least one of the electric characteristics, the inter-electrode resistance and the characteristics of resistance change during continuous spot welding by using the identical weld electrodes;
estimating the number of spots or the duration of spot welding until the successful nugget will not be formed according to the record in the continuously recording step;
automatically controlling to change subsequent weld conditions when the estimated number or duration reaches a predetermined standard;
secondarily compensating the forming of nugget by activating an additional back-up system when it is determined that the nugget is not formed according to the record in the continuously recording step or due to an unexpected accident occurred in the series of the steps; and conveying the initial members between the steps, the steps being adapted to constitute a production line totally controlled by a host computer.
According to a second aspect of the invention, there is provided a spot welding system effecting the steps of:
placing a resistance increasing material at a predetermined position on the bonding surface of one of the initial members;
overlapping the other of the initial members on the one of the initial members while clamping the resistance increasing material between the initial members;
positioning a center axis passing through the pair of weld electrodes over substantially the center of the resistance increasing material clamped between the bonding surfaces to apply a predetermined pressure by the weld electrodes to the resistance increasing material and the initial members;
flowing a weld current having a predetermined value between the weld electrodes in a predetermined time;
detecting electric characteristics with respect to the weld electrodes in the predetermined time;
calculating an inter-electrode resistance based on the detected electric characteristics and calculating characteristics of resistance change based on the inter-electrode resistance;
determining success or failure in forming a nugget between the bonding surfaces by comparing the characteristics of resistance change with a predetermined standard;
automatically changing weld conditions upon the determination of failure in the determining step and primarily compensating the forming of the nugget;
comparing the predetermined standard with characteristics of resistance change additionally calculated after the compensating step and secondarily determining success or failure in forming the nugget; and
recording the determination of failure in forming the nugget in the secondarily determining step.
According to a third aspect of the invention, there is provided a spot welding system effecting the steps of:
placing a resistance increasing material at a predetermined position on the bonding surface of one of the initial members;
overlapping the other of the initial members on the one of the initial members while clamping the resistance increasing material between the initial members;
positioning a center axis passing through the pair of weld electrodes over substantially the center of the resistance increasing material clamped between the bonding surfaces to apply a predetermined pressure by the weld electrodes to the resistance increasing material and the initial members;
flowing a weld current having a predetermined value between the weld electrodes in a predetermined time;
detecting electric characteristics with respect to the weld electrodes in the predetermined time;
calculating an inter-electrode resistance based on the detected electric characteristics and calculating characteristics of resistance change based on the inter-electrode resistance;
determining success or failure in forming a nugget between the bonding surfaces by comparing the characteristics of resistance change with a predetermined standard;
automatically changing weld conditions upon the determination of failure in the determining step and primarily compensating the forming of the nugget;
comparing the predetermined standard with characteristics of resistance change additionally calculated after the compensating step and secondarily determining success or failure in forming the nugget;
recording the determination of failure in forming the nugget in the secondarily determining step;
continuously recording at least one of the electric characteristics, the inter-electrode resistance and the characteristics of resistance change during continuously spot welding;
estimating the number of spots or the duration of spot welding until the successful nugget will not be formed according to the record in the continuously recording step; and
automatically controlling to change subsequent weld conditions when the estimated number or duration reaches a predetermined standard.
It is desirable that the changing the weld conditions for the primarily compensating step includes prolonging the duration for flowing the weld current.
When the successful nugget is not formed according to the record in the continuously recording step or due to an unexpected accident, it is preferred to secondarily compensate for the failure by activating an additional back-up system.
It is desirable that the estimating step estimate the number of spots or duration until a sufficient nugget will not be obtained by comparing a predetermined standard with a resistance value variation characteristic during the successive spot welding. It is also desirable that the alteration of the weld conditions in the controlling step be an automatic grinding of the weld electrodes.
It is also possible in the estimation step to estimate the number of spots or duration until a sufficient nugget will not be obtained by comparing a predetermined reference with a frequency of irregular current conducting during successive spot welding. The alteration of the weld conditions in the controlling step may also be an increase in the electrode pressure (weld force).
The alteration of the weld conditions in the controlling step may also be the extension of the predetermined current conducting time of the weld current.
The alteration of the weld conditions in the controlling step may also be an increase in a predetermined electric current value.
It is possible in the overlapping step to use a spacer to ensure a gap between the bonding surfaces of steel sheets to be bonded.
It is desirable that the resistance increasing material be one which, in the pressurizing step, leaves a part of the gap around the spacer between the bonding surfaces such that the bonding surfaces may partially contact each other.
It is desirable that the resistance increasing material leave, in the pressurizing step, a part of the gap around the spacer between the bonding surfaces such that the bonding surfaces may partially contact each other, and it is desirable that the retained gap have such a size as to let the melted or evaporated zinc escape.
The resistance increasing material is a mixture of poorly electrically conductive particles and an adhesive material. It is preferred that the particles function as a spacer, and the adhesive material foam or the adhesive force be increased, when heated or aged.
The resistance increasing material may also be a perforated tape having an adhesive coated on its opposing faces.